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(S-9) Nuclear Weapons

The preceding web page on nuclear power, written two years ago, deliberately avoided the subject of weapons. The world is changing, and reluctantly this part was added, in the belief it is be better to know than to remain ignorant.

To web-users led here by a search engine: this web page is the 3rd in a sequence discussing nuclear energy at an elementary level, part of a very large web course "From Stargazers to Starships" on astronomy, Newtonian mechanics, the Sun and spaceflight. . The two preceding it are The Energy of the Sun and Nuclear Power.

Explosive release of Fission Energy

The fission of uranium currently provides electrical power in several countries. Some day it may do so on a much larger scale, if the world can agree on ways to prevent its misuse (see below) and on the safe disposal of fission products.

However, it also has a second aspect--the making of nuclear bombs. Instead of gradually "burning up" U235 or plutonium (the main nuclear fuels), producing heat for generating electricity, a bomb releases its energy very abruptly, creating an intense concentration of heat. Even a few kilograms (of which just a small part undergoes fission) can destroy a city.

Such a sudden release is not easily achieved. Rather sophisticated technology is needed, otherwise heat released early in the fission "chain reaction" blows the fuel apart and stops the process. A nuclear reactor can never explode like a bomb: the most it can do is explode like a steam boiler without safety valves, or more likely, its fuel might melt down into expensive slag, as happened on Three Mile Island. Malfunctioning reactors can certainly be dangerous, since they hold intensely radioactive fission products (remember Chernobyl!). However, they cannot become nuclear bombs.

A second reason also exists: the process used in power stations is not suitable for building bombs. Each neutron released by the fission of a nucleus must bounce around inert matter (e.g. water or carbon) and slow down, before it can initiate a new "thermal" fission. The extra matter interferes with the explosive process, and the slowing-down process, fast as it is, stretches out the energy release.

Fission can, however, proceed much more rapidly by a different process, using "fast" neutrons. Freshly created from the fission process, they initiate another fission before moving very far. "Fast fission" makes bombs feasible (as well as "fast breeder" reactors, of which France has built two). Its chain reaction is damped by the U238 isotope, so that unlike the fuel in a power station, where even natural uranium (0.7% U235) can be used, the one used in a bomb must be heavily enriched in U235 ("weapons grade uranium"). Or else, the artificially produced element plutonium is used, extracted as a by-product of nuclear reactors.

Enrichment is a difficult and expensive process, using gas centrifuges or large magnetic separators (methods used by Iraq prior to 1991), or else large gas-diffusion arrays. Plutonium, on the other hand, requires only chemical methods to separate it from the intensely radioactive fission products in spent reactor fuel. Because of the deadly radiation, such separation is always done by remote control. The possibility that nuclear bombs may be constructed from plutonium extracted from commercial power plants has been the main motivation of international efforts to control the spread of nuclear power technology.

Such radioactivity can contaminate the location where the bomb explodes, to a range of perhaps a hundred feet or a few hundred feet. Coming close to such radioactivity is not likely to kill or seriously harm anyone. Much more harmful is ingesting it with food or air, so that it ends inside the body. Prompt medical treatment can remove most of it, but the main point would be to instill fear, in a visible and conspicuous way

The contamination must be cleaned up: failing to do so would create a long term health hazard. If a dirty bomb were exploded at a national shrine or monument, or at a focus of public activity, that place would have to be closed down, at the very least temporarily, and expensive clean-up would have to start, all these very visible actions. Furthermore, the general public is unfamiliar with nuclear physics and dreads anything tied to it. It is likely to react in fear.

Nuclear waste is usually well-guarded, since it is in the interest of any government to keep dangerous radioactivity away from its own citizens. Yet severely stressed societies exist, which lack the resources to prevent determined terrorists from breaking into depositories of nuclear waste, or from finding ways to quietly steal some of their contents. It is yet another danger we must address in this day and age.

Books about Nuclear Weapons

Out of the huge existing literature, here are a few samples. Be warned they my be out of print, though you might find them in libraries:

The Making of the Atomic Bomb by Richard Rhodes, a thorough history, quite large and very well written. Simon and Schuster, 1987.
Dark Sun by Richard Rhodes, a continuation of the above story--the nuclear bomb effort of the Soviet Union (including its spying on the US) and the hydrogen bomb.
Simon and Schuster, 1995.
The Effects of Nuclear Weapons, edited by Samuel Glasstone, published 1962 by the U.S. Government Printer for the U.S. Atomic Energy Commission. Detailed, full of graphs and figures.
Atomic Energy for Military Purposes, Henry DeWolf Smyth, Princeton U., 1945. The "Smyth Report," the first report published soon after the revelation of the US effort to produce the first nuclear bomb.